95 Mercury Overheating

95 Mercury Sable 3.8 overheating: In hot weather my Sable temp guage rises to the mid-way point when running the AC. The car stumbles and the engine quits. After I let it cool off and drive it again with the heater on full blast, and not stopping, the car runs without quitting.
It's got a new radiator and anti-freeze fluid, a fairly new thermostat and water pump. Although I don't see any leaks, from time to time I have to add fluid. Could it be the Constant Control Relay Module for the fan, or, the Engine Coolant Temperature Sensor? Maybe it needs a new radiator cap? What do your suggest I check? Thanks!